1. Field of the Invention
The present invention relates to a water content determination apparatus that determines water content contained in a sheet of paper used for an image forming apparatus such as a copier and a printer, an image forming apparatus, a control method, and a program for implementing the method.
2. Description of the Related Art
Conventionally, an electrophotographic image forming apparatus forms an image by developing a latent image formed on a photosensitive member with toner and transferring a toner image onto a sheet of paper. It is known that as for an electrographic image forming apparatus, the water content contained in a sheet of paper to which toner adheres has some influence on image quality. Some image forming apparatuses of this kind have adopted a method to detect water content contained in a sheet of paper, and then to feed back controlled variable based on the result to the various kinds of controls regarding image forming.
As for a method for detecting water content contained in a sheet of paper, generally, humidity (water content in the air) in the air inside and outside an image forming apparatus is measured with a humidity sensor or the like, and the water content contained in a sheet of paper is relatively estimated based on the measurement value.
However, in the water content estimation method with a humidity sensor, the water content contained in a sheet of paper is not detected directly, but the water content contained in the sheet of paper is estimated based on the measured environment (water content in the air). For example, when a cassette for an image forming apparatus is loaded with sheets of paper brought from another place in a different environment, the sheets of paper neither can absorb the water content that is considered to be contained in the environment, nor let off the water content contained in the sheets of paper. Therefore, a measurement value measured by a humidity sensor measuring the environment in which an image forming apparatus is installed does not always estimate the water content contained in a sheet of paper. Accordingly, it is difficult to obtain data to control an image forming apparatus optimally in the water content estimation method with a humidity sensor.
To solve the above problem, a way of measuring moisture in a sheet of paper directly by measuring the state of the sheet of paper when voltage is applied to it is proposed (refer to Japanese Laid-Open patent publication (Kokai) No. 2002-365976, for example). Furthermore, a way of measuring water content contained in a sheet of paper based on change of electrostatic capacitance while a pair or a plurality of pairs of electrodes are located from across a paper path in such positions as a pair or a plurality of pairs of electrodes sandwich sheets of paper in between (refer to Japanese Laid-Open patent publication (Kokai) No. 2003-065994, for example).
However, in the above conventional water content measurement method with an electrostatic capacitance way, measurement data changes dependent on temperature of the environment where an image forming apparatus is installed. Therefore, there has been a problem that precise water content contained in a sheet of paper cannot be determined because measurement data is different depending on the environment where an image forming apparatus is installed (for example, whether the air-conditioning is kept constant in a room, whether temperature change is great in a room, or whether the place is a cold area or a warm one)